Chloride and osmotic contractures in skinned frog muscle fibers

Summary Single contractures were elicited in segments of skinned frog muscle fibers when the segments were moved from relaxing-loading solutions to various test solutions. The effective test solutions produced an increase in the concentration of chloride ions in the myofilament space, [Cl]_ms, and/or presumably caused the sarcoplasmic reticulum to undergo a change in volume. The contractures were quantified in terms of their maximum tension and time-integral. Two outer segments from each fiber underwent a contracture in a control solution (chloride ions were substituted for all of the methanesulfonate ions in the relaxing solution). The mean values of tension and area in the control contractures of each fiber were divided into the corresponding values from a test contracture obtained in the central segment of the same fiber. Test contractures obtained upon increasing [Cl]_ms and increasing the product, [K]_ms×[Cl]_ms, were compared to contractures that were obtained by increasing [Cl]_ms while keeping [K]_ms×[Cl]_ms constant. The former contractures were greater in magnitude for a given [Cl]_ms. Whereas the former solutions may have caused an increase in the volume of the sarcoplasmic reticulum and altered the electrical potential across the membranes of the sarcoplasmic reticulum as well, only a change in potential was presumed to have occurred in the latter solutions. Other types of contractures were investigated to show that both swelling of the sarcoplasmic reticulum and changes in the electrical potential of its membranes can cause release of calcium ions and elicit contractures in skinned fibers.